drm/amdgpu: Add helper function amdgpu_ring_set_preempt_cond_exec

can preempt the ring by setting cond_exec to false

Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>